1. Field of the Invention
The present invention relates generally to a projection type exposure apparatus and method and, more particularly, to a structure of a lens barrel of a projection optical system.
2. Related Background Art
FIG. 7 illustrates a structure of a lens barrel of a prior art lens projection (refraction) type projection optical system of a projection type exposure apparatus. As shown in FIG. 7, the conventional lens projection type projection optical system transfers a pattern formed on a photo mask 4 like, e.g., a reticle onto a projected substrate 5 such as a wafer. This lens projection type projection optical system has a single lens barrel 1. A bottom of the lens barrel 1 is formed with an opening 6 and a protrusion member 7 protruding inward. An upper portion of the lens barrel 1 is also formed with an opening 8 into which a ring-like holding member 3 is screwed.
Further, the lens barrel 1 incorporates a plurality of lens frames 2 for housing optical elements (lenses) L, respectively. An optical axis of each lens L housed in the lens frame 2 is located to coincide with an optical axis AX of the lens projection type projection optical system.
Thus, the structure of the lens barrel of the prior art lens projection type projection optical system entails such a configuration that the single lens barrel accommodates all the lens frames.
In the case of assembling the prior art lens projection type projection optical system having the above-discussed construction, the individual lens frames 2 housing the lenses L are sequentially inserted down into the interior of the lens barrel 1 via the upper opening 8. The lens frame 2 which has been thus inserted down is supported on the protrusion member 7 formed at the bottom of the lens barrel 1, thus laminating the lens frames 2 in the interior of the lens barrel 1. When housing all the lens frames 2, the holding member 3 is fitted in the opening 8, thus completing the assembly thereof.
Generally, the lens projection type projection optical system needs to make optical adjustments such as an adjustment of a lens-to-lens distance, an adjustment of an inclination (hereinafter termed a [tilt]) to the optical axis AX of each lens and an adjustment of an optical axis eccentricity of each lens with respect to the optical axis AX. In the conventional lens projection type projection optical system having the above-mentioned construction, such optical adjustments are required to be performed after removing the respective lens frames 2 out of the lens barrel 1. The optical adjustments are also required to be effected while seeing a result of image formation through the whole system, and, therefore, the optical adjustment can be carried out only in such a state that the whole construction is accomplished by incorporating all the lens frames 2 in the prior art lens projection type projection optical system.
As described above, in the conventional lens projection type projection optical system, all the optical elements are housed in the single lens barrel, thus constructing one assembly unit. Accordingly, if a certain specified optical element needs the optical adjustment, a removal of this specified optical element out of the lens barrel implies exactly a decomposition of one assembly unit. Further, as explained above, the optical adjustments can be conducted only in the state where the whole optical system is reconstructed by incorporating all the lens frames, and, hence, those operations of the decomposition and the reconstruction (reassembly) are required to be repeated for the optical adjustment of even one lens element. There exists such a drawback that an efficiency of the optical adjustment remarkably worsens with a larger number of constructive lens elements.
Further, a structure of the lens barrel of a mirror-lens projection (catadioptric system) type projection optical system is much more complicated than in the conventional lens projection type projection optical system described above. The mirror-lens projection type projection optical system also has the same drawback inherent in the optical adjustment as that of the conventional lens projection type projection optical system.